Tag: saturation

We’ve all heard about glide, but what is it really and how does it affect our system?

Glide, or temperature glide, is the difference between the bubble point and the dew point of the zeotropic refrigerant mixture.

Well that wasn’t very helpful, was it? All we did was introduce new terms without defining them and further confused the issue.

So, let’s start with zeotrope or zeotropic mixture. A zeotropic mixture is a chemical mixture that never has the same vapor phase and liquid phase composition at the vapor-liquid equilibrium state. Still unhelpful? I thought so, too, so let’s look at what it means to us rather than what the books say.

A zeotrope, is a refrigerant mixture or blend that boils across a range of temperatures at any given pressure. So, unlike water that boils at a constant temperature of 212°F at atmospheric pressure, a zeotropic mixture will boil between across a range of temperatures at that same single pressure. Using r407a as an example, at atmospheric pressure, the liquid would begin to boil at -49°F and will continue to boil until the last droplet boils away at -37.5°F. I know that it’s kind of weird to think of the process of boiling like that, but that’s what is happening with a zeotrope. Boiling takes place over a range of temperatures. That temperature range is called the glide.

Now that we’ve got a basic concept that we can work from, we can start to understand glide and ultimately get to how it affects a refrigeration system. Let’s start with bubble point. Since we should have a solid understanding of states of matter and the transition between liquid and vapor, let’s assume we have r407a refrigerant in a 100% liquid state at 140 psig. If we start at 66°F, we’ll be just slightly subcooled which is a perfect starting point for this example.. If we start to add heat and raise the temperature of our refrigerant while holding our pressure constant, a single bubble will appear in the refrigerant as it begins to boil. That point is called the bubble point. For our purposes, we can define the bubble point of a zeotropic refrigerant blend is the point at which the first bubble appears. Still making sense? I hope so.

Continuing with our r407a at 140 psig example, we’re going to continue to add heat to the refrigerant with the same constant pressure. The refrigerant continues to boil, but as the mixture of refrigerant changes, the boiling point changes, slowly rising as the liquid boils away. Eventually, we will have added enough heat to reach a point where one last droplet exists, that point is called the dew point. Like we did with bubble point, let’s state an operating definition for bubble point. The dew point is the point at which the last droplet of liquid evaporates. For our example, that temperature is 75.5°F or very near that. Since it’s boiling over a range of temperatures, it is also true that the refrigerant condenses over the same range of temperatures as we remove heat from it. That will happen in reverse of the process I just described.

What does this mean for the service guy?

Obviously, these different values affect our superheat and subcooling readings. Since the dew point is the point where the last droplet of liquid boils off, we need to know that value to measure and calculate superheat. Similarly, with the bubble point, we need that to calculate subcooling. These are the values found on the PT charts and that are programmed into your digital manifold gauges.

In refrigeration work, evaporator coil temperature can be used for a number of things. Most commonly, we will use it to control fixture temperature and to terminate defrost. It used to be simple to know what our evaporator temperature is. We looked at the gauge and transferred that number to a PT chart. We can no longer look at our evaporator pressure and know what our corresponding evaporator temperature is quite the same way.

Let’s look at numbers… say the manufacturer says that you need an 18°F coil temperature. With R22, you simply look at your trusty PT chart, find 40.9# and work from there. Easy enough, right? Now, let’s look at the same coil with r407a. We have 2 points that are 18°F. The dew point (40#) and the bubble point (52.5#), so which one do we choose? The correct answer winds up being neither one. Between manufacturer’s recommendations and field experience, I’ve found it best to use something closer to the average of dew and bubble point to find the actual, functional temperature of the evaporator.

52.5+40 = 92.5. 92.5/2=46.25

Looking at a PT chart, this shows us 13°bubble point and just over a 23° dew point. If you look, 18° will land right about in the middle. This isn’t always a perfect setting, but it’s as good a place to start as you can find. Set the control valve there and fine-tune it as needed to get the performance that you need. If we need to use a pressure reading to terminate defrost, we will need to reference bubble point because it is the colder of the two temperatures and will ensure a complete defrost. If we used dew point, the inlet of the evaporator would be several degrees colder than the outlet and frost may still very present.

Like we often do in these tech tips, we will start with the common and more practical explanation of saturation and then move to the more technical and nerdy explanation later.

When we say “at saturation” or “saturated” in the HVAC/R trade we are generally referring to refrigerant that is in the process of changing from liquid to vapor (boiling) in the evaporator or vapor to liquid (condensing) in the condenser.

We generally look at a set of gauges or find the temperature on a PT (Pressure – Temperature) chart that matches a particular refrigerant and pressure and we call that the saturation temperature.

So when a tech connects gauges to the liquid line (high side) of a system and they look at the needle they will refer to the pressure in PSI and the temperature for the particular refrigerant as saturation temperature. On the gauge above the refrigerant in the system is R22 (green scale) they would say that the pressure is 200 PSI and the saturation temperature is 102°F.

To be even more specific, a tech might say that the condensing temperature of this system is 102°F because the saturated state is occurring during the process of condensing in this particular case.

From a practical standpoint in a refrigeration circuit when we say saturation we are referring to –

the pressure and temperature a refrigerant will be if both liquid and vapor are present at the same time and place

One of the most common cases where we will see refrigerant at saturation is inside systems that are off as well as inside a refrigerant tank. If you were to connect a gauge to tank (like this Testo 550 shown) the refrigerant pressure inside the tank will be equal to the pressure that correlates to the saturation temperature of the tank (I know that’s a mouth full but it’s really pretty simple).

In the case of the refrigerant shown above the room temperature is 71.9°F and the refrigerant is R-422D. All I had to do was connect the Testo 550 and select R422D and the saturation temperature (show above the psi on the right) is EXACTLY 71.9°F. In this case, we can say the saturation PRESSURE of R422D is 136.8 PSI at 71.9°F or that the saturation TEMPERATURE is 71.9°F at 136.8 PSI.

Either way, what are saying is that there is both liquid and vapor present inside the tank so it is at SATURATION or in the saturated state if you would rather. So as techs we see refrigerant at saturation pressure and temperature when the system is off, inside a tank and when it is in the midst of boiling in the evaporator or condensing in the condenser.

Now for the more in-depth explanation

I will warn you that this is a bit of a beating around bush explanation, but I’m writing the explanation I wish I had been given early on… so be patient young grasshopper.

Let’s start with a dictonary definition of saturation –

The state or process that occurs when no more of something can be absorbed, combined with, or added.

So when something is “full” and can hold no more of something it is said to be saturated, like a sponge saturated with water, or air saturated with water vapor or a in this case, a liquid saturated with kinetic energy.

Many (including Wikipedia) will define saturation as the boiling point of a liquid. This definition is correct but can lead to a misunderstanding. Just because a liquid is at its boiling point doesn’t mean it is actively boiling. The refrigerant in an air conditioner is technically at the boiling point when the system is completely off. Refrigerant in a tank is at saturation (so long as it has some liquid in the tank) even though the refrigerant is static (nor flowing).

In nature, gasses (vapor) and liquids are free to move around and interact with one another with the predominant pressure being atmospheric pressure (14.7 psia at sea level).

You may have wondered why water exposed to the air will evaporate even though it has not reached the boiling temperature? This is because the temperature of a substance is the AVERAGE kinetic energy of the molecules in a substance not the specific kinetic energy of every single molecule. While there may not be enough energy for the entire substance to boil, there is enough energy in a few of the molecules to break free from the surface.

This is why when sweat evaporates off of your skin your skin cools. The highest energy molecules are leaving and taking themselves and their high energy ways with them!

Translation – Some molecules have more energy than others and are able to escape the liquid form out in nature and we call this evaporation. This evaporation can be measured but it happens below the boiling point and when a substance is uncontained it results in less liquid remaining.

Translation of the Translation – If you leave water out in a pan it will eventually disappear even when it isn’t boiling

Now if you put a liquid in a jar and screw the lid on, some of the molecules will escape the liquid bonds and fill the void in the jar until pretty soon the jar will be at equilibrium (static) pressure with an equal number of molecules condensing back into the liquid as those that are escaping. The more active the molecules in the jar the more pressure there will be in the jar. Since the definition of temperature is the average kinetic energy (energy of motion) of the molecules you can translate that as “The hotter the jar the higher the pressure” or “The higher the pressure the hotter the jar”.

Different liquids have a more or less tendency to escape the liquid form (evaporate), liquids that have a very high tendency to escape will evaporate more quickly and have a higher “vapor pressure” and are also said to be more “volatile”. Alchohol or gasoline are liquids that are more volatile and have a higher vapor pressure at atmospheric pressure than water and disappear quickly even when the ambient temperature is below their boiling point.

Some Liquids (like vacuum pump oil for example) have a very low tendency to evaporate and are said to have very low volatility and a low vapor pressure.

Liquids with low boiling temperatures (like most refrigerants) are very volatile and have a higher vapor pressure than liquids that remain a liquid at atmospheric pressure. We know that refrigerant does more than evaporate at atmospheric pressure and normal atmospheric temperature, it literally BOILS.

A liquid boils when the vapor pressure of the liquid matches the atmospheric pressure. At that point the liquid molecules begin to break free rapidly and if they are uncontained they will simply fly away like water vapor out of an open pot.

If the molecules are boiling and contained they will begin increasing the pressure as they boil until the temperature of the liquid no longer increases and it hits equilibrium between the vapor pressure of the liquid and the pressure inside the vessel (tank, pressure cooker etc..).

Once the vessel is allowed to reach a state of perfect equilibrium it may no longer be boiling but it can still be at the boiling point, that exact POINT of equilibrium between vapor pressure and temperature is the SATURATION POINT.

So long as the pressure remains constant on a boiling or static vapor/liquid mixed substance we can say that it is at saturation temperature because it remains at the same temperature until either

The pressure changes

The substance is fully boiled

But it is important to remember that it is the vapor pressure of a liquid substance being equal to the pressure around it that results in saturation and then boiling or in the opposite direction, condensing.

Also… Evaporators should be called boilerators but I’m doing being nerdy for now.

We have discussed DTD (Design Temperature Difference) quite a bit for air conditioning applications, but what about refrigeration? Let’s start by defining our terms again

Suction Saturation Temperature

Saturation temperature is the temperature the refrigerant will be at a given pressure if it is in the process of changing state. This change of state would be from liquid to vapor (boiling) in the case of the low side (evaporator / suction line). When we look at saturation temperatures instead of pressures we can use similar rules and we will see similar saturation temperatures across all refrigerants when the application is the same. Experienced HVAC and refrigeration techs pay far closer attention to the saturation temperatures than they do pressures.

Evaporator TD and DTD

Evaporator TD (temperature difference) is the measured difference between the suction saturation temperature (evaporator boiling temperature) and the box temperature. DTD (design temperature difference) is the designed or expected TD.

Delta T

Many A/C techs will confuse TD with Delta T. Delta T is the difference between the evaporator AIR temperature entering the coil to the air temperature leaving the coil. The Delta T will vary based on the humidity in the box where TD will not.

Target Box Temperature

The temperature the refrigeration box should maintain when the system is operating properly

Superheat

The increase in temperature between the suction saturation temperature and the suction line temperature leaving the evaporator. Superheat is the temperature (sensible heat) gained between the point that all of the liquid boiled off in the evaporator coil and the suction line at the outlet of the coil. in refrigeration, like HVAC 10°F(5.5°K) of superheat is average with a range from 3°F to 12°F(1.65°K – 6.6°K) depending on the equipment type (10°F(5.5°K) for med temp, 5°F(2.75°K) for low temp, 3°F(1.65°K) for ice machines ).

Hot Pull Down

Refrigeration equipment is unlike HVAC equipment in that the evaporator will spend most of its life running in a very stable environment with minimal fluctuation in the box temperature.

On occasion a refrigeration system will see a huge change in load in cases where it was off and needs to “pull down” the temperature, or when doors are left open or when a large quantity of warm product is placed in the box. When a piece of refrigeration equipment is in hot pull down it cannot be expected to abide by the typical DTD or superheat rules and must be allowed to get near the design box temperature before fine adjustments are made to the charge, TXV superheat settings or to the EPR (Evaporator Pressure Regulator) if there is one.

Design Temperature Difference (DTD)

In air conditioning applications a 35°F DTD is a good guideline for systems that run 400 CFM(679.6 m3/h) of air per ton of cooling (12,000 btu/hr). In refrigeration the DTD is much lower than in air conditioning.

There are several reasons for this but one big reason is the desire to maintain relatively high relative humidity levels in refrigeration to keep from drying out and damaging product. Keep in mind that NOTHING is a substitute from manufacturer’s data but here are some good DTD guidelines for traditional / older refrigeration equipment. Keep in min dthat the trend is toward lower evaporator TD on newer equipment.

You then subtract the DTD from your box temperature / return temperature to calculate your target suction saturation. You can then use this target saturation / DTD and compare it to your actual measured saturation and DT once the box is within 5°F – 10°F(2.75°K – 5.5°K) of it’s target temperature to help you set your charge, TXV and EPR as well as diagnose potential airflow issues when compared with suction superheat and subcooling / clear site glass.

For Example –

If you have a medium temp walk-in cooler with a 35°F(1.66°C) box temperature you would expect to see a suction saturation of 25°F +/- 3°F

When doing a quick inspection of a piece of refrigeration equipment without gauges you can use this data to do the following calculation –

In this particular case logic tells us that the suction line could be no WARMER than 35°F(1.66°C) because that is the temperature of the air the refrigerant is transferring its heat to. However by the time you factor in the the accuracy of your box thermometer and line thermometer and the assumed saturation temperature you would still expect a 35°F(1.66°C) suction line temperature +/- 3°F(1.65°K)

For a -10°(-23.33°C) box, low temp reach-in you would calculate it this way

Clearly, this is NOT the way to commision a new piece of equipment or to benchmark a system you haven’t worked on before, but it can give you a quick glimpse at the operation of a piece of refrigeration equipment without attaching gauges, especially on critically charged or sealed systems.

The best practice is to know the equipment you are working on, read up on it and properly log benchmark data the first time you work on a piece of equipment or during commissioning.

It should also be noted as Jeremy Smith pointed out, in recent years TD’s have been decreasing as manufacturers seek higher efficiency through higher suction and lower compression ratios.

This means that TD’s as low as 5 can be designed into some units but keep in mind… the suction line can still be no warmer than the box so as DTD drops so does superheat and the critical nature of expansion valve operation.

Refrigeration Technologies

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